A soil and foliar nutrient analysis was carried out in the Hawaiian
Metrosideros rain forest for the purposes of elucidating a previously
published physical habitat classification and for finding an explanation for
the widespread canopy dieback, which is not caused by a biotic agent in this
ecosystem. Soil elements analyzed were C, N, P, Ca, AI, Mn, Fe, and other
parameters such as pH. Foliar analysis was restricted to N, P, Ca, Mn, and
Fe and assessed only for the canopy M. polymorpha and its major associate,
the tree ferns, in the undergrowth. It was found that the earlier recognized
two-way breakdown into oligotrophic and eutrophic nutrient regimes had to
be enlarged for the habitat classification to a mesotrophic category. Young,
deep-ash soils were recognized as mesotrophic on account of their moderate
pH and high Ca levels, but they showed severe limitations in available nitrogen
and phosphorus that were reflected also in the foliage. Intermediate-aged
(1000 yr) ash soils were found to be nutritionally well supplied and balanced
and thus considered eutrophic, while older ash soils were designated as oligotrophic
on account oftheir extreme acidity and associated higher and potentially
toxic levels of soluble AI, Mn, and Fe. Except for the mesotrophic 'a'a lava site,
other shallow rock outcrop habitats on pahoehoe were recognized as oligotrophic,
because of very low available N and high acidity and, in the poorly
drained sites, also high levels of potentially toxic metals. The bog habitats had
the highest levels of potentially toxic metals, but surprisingly moderate levels
of available N and P. The canopy dieback phenomenon cannot be entirely
explained from the nutrient imbalances found in most of the substrate types,
but these imbalances are considered contributory in the sense that they become
increasingly stressful with stand development, thereby decreasing tree vigor
and predisposing stands to dieback.